I do think it is ok, and good actually, to go back and forth. Blocking in proportions in CAD, then ideating overtop in sketches. A lot of times the mechanical engineers I work with will send me STEP files, I’ll screen cap them and sketch overtop in photoshop or sketchbook pro and iterate a bunch of detail ideas back. I have one project I just wrapped up where the client’s ME group would send me weekly CAD progress and I’d sketch out detail refinements for them to work on in the next week. I think it is important to remember that the design process is not linear. sketching might lead to some CAD and some mockups and back to sketches and then back into CAD and round and round. You need to use the right tool to advance the idea in that moment.